Those who say that Christ was lapsible will usually say that the basis for this is the fact that pre-fall Adam
was lapsible, and Christ was the second Adam.
As for Adam, they say this (from the Westminster Confession of Faith): "Man, in his state of innocency, had freedom and power to will and to do that which is good and well-pleasing to God; but yet mutably, so that he might fall from it."
So they say that Christ, as the second Adam, was also in a state of innocency/sinlessness, but yet mutably, so that he might fall from it. They say that both were under a "covenant of works."
What these people do not recognize is the specific way in which Christ was the second Adam. Instead, they go further than Scripture, and in doing so, they blaspheme.
The following is the truth of the parallel between Adam and Christ:
Adam was a representative of the entire human race, and when he sinned, his sin is imputed to all whom he represented.
Christ was a representative of a certain people, and when He lived a perfect sinless life, his righteousness is imputed to all whom He represented.
This is the parallel. Now if people want to go further and say that Adam and Christ are entirely alike, then they would have to say the following:
(1) Either Adam was a God-man, or Christ was solely man and not God.
(2) Either Adam was impeccable, or Christ was lapsible.
When people start getting into hypotheticals, they get into heterodoxy. We've seen a few of these hypotheticals before, such as:
(1) If Adam would have remained sinless (after a probationary period), then he would have entered everlasting bliss.
(2) If Christ would have died for the reprobate, then they would be saved.
(3) If God had decided to save people another way other than through the death of Christ, He could have done so.
(4) Christ died for the reprobate in case they would believe.
(5) Christ could have sinned.
(6) Blah, blah, blah.
Let's stick to the Word of God. All else is vain speculation.